Euglena gracilis provides a unique system for the study of the role of 3', 5' -cyclic adenosine monophosphate (cAMP) in a unicellular organism which has characteristics in common with both plants and animals. The proposal is concerned with the following problems. 1) Where is adenylate cyclase located in the Euglena cell and what are its characteristics? I have evidence from cytochloroplast envelope. Perhaps there are two distinct adenylate cyclases, each responding to different stimuli and each having its own distinctive characteristics, 2) Where is phosphodiesterase located in Euglena cells and what are its characteristics? My cytochemical experiments indicate that there is phosphodiesterase in both the pellicle and within the chloroplasts of Euglena. Others have found phosphodiesterase in Euglena chloroplasts by biochemical techniques. 3) What is the role of cAMP in the Euglena cell? If there are two sites of adenylate cyclase, one in the pellicle and one in the chloroplast, then CAMP may be produced in two different compartments and function differently in each compartment. In one compartment, cAMP may stimulate a protein kinase as it does in animal systems; a cAMP-stimulated protein kinase has been reported in Euglena cells. In the other compartment, the chloroplast, cAMP may act in a manner similar to which it functions in bacteria and influence chloroplast development. Evidence from three laboratories supports the role of cAMP in Euglena chloroplast development. Could repression of Euglena chloroplast development by glucose operate by a mechanism similar to the mechanism of catabolite repression found in bacteria grown on a glucose-containing medium? 4) What is the influence of light and nutrients on adenylate cyclase in Euglena and on the level of cAMP in the cell? 5) Does Euglena possess guanylate cyclase? If so, where is it located and does its activity fluctuate during the life cycle of the cell? My experimental approach to the above problems is the following: biochemical assays on specific fractions from broken Euglena cells, radioimmunoassays, and further cytochemical experiments.